laganas_OLI_20150801

GSD 15m pan-sharpened
The data

TCC most variable atmosphere	TCC deglinted Note the low Blue radiances along the western waterline of Zakynthos island: what's that? This happens at various locations in this scene
Deglinting along Profile_Yellow	Glint regressions are excellent Deglinting is excellent Adjacency effect is fairly strong, several kilometers wide

Calibration under RED ROI

Calibration Blue vs Green

Big star: brightest substrate is set to Coral Sand
BPL pixels: clearly display as a straight line
Reflectance of coral sand is ~2.7 times brighter than this bottom
K_blue/K_green=0.33 and K_coastal/K_green=0.35
- this is Jerlov water type OI+0.9
- this is very-very clear, just below OIA
Deepest bottom detection for this bottom: ~28 m
BUT deepest bottom detection for coral sand would be ~47 m
Shallowest pixels are at ~7.5 m
- this sandy slope stretches from 7.5 m down to 28 m
Very few pixels in this ROI exhibit darker bottoms
Land pixels: they provide a clear view of the SOIL Line assumption
- Lw_coastal~=26 La_coastal ~=70
- Lw_blue ~=20 La_blue ~=47
- Lw_green ~= 2 La_green ~=27
- Lw_pan ~= 1.5 La_pan ~=32

Calibration Coastal vs Green

Calibration Blue vs PAN

Calibration under BLUE ROI

Calibration Blue, Green, Red, NIR

Small crosses: I have added the BPL pixels of Red_ROI
Unchanged: calibration settings are unchanged
Identical: optical calibration for the water is quasi identical in both ROIs
Very dark: brightest bottoms at Red_ROI are three times brighter than at Blue_ROI
- this makes for very dark sandy bottoms at Blue_ROI: only 1/8 of the brightness of coral sands!

Calibration Coastal, PAN, Red, and NIR

Ready for Modeling

Retrieved depth in centimeters see full scene see legend	CC BOA WCC B=Coastal G=Green R=Blue
B average bottom brightness

**Insight on the uncertainty on fake depth over Land areas**
Conclusion by Fabio Favoretto et al, 2017 In conclusion, the findings suggest that 4SM is as accurate as the commonly used Stumpf’s method, the only difference being the independence of 4SM from previous field data, and the potential to deliver bottom spectral characteristics for further modeling. 4SM thus represents a significant advance in coastal remote sensing potential to obtain bathymetry and optical properties of the marine bottom.
ZLand In order to test 4SM against spectral variations of the bottom substrate, I can enforce a depth over land to apply the water column attenuation from image calibration, then process these "artificial shallow" pixels to see how well/bad this depth is retrieved.
Result of this exercise As demonstrated by the profiles below, from very shallow to very deep, the algorithm yields a surprisingly good estimation of depth: there is hardly any increase in uncertainty as depth increases.
Noise But, wait: this does not include the quantization noise (mmm...)! I tried including 8_bits quantization of the computation of "artificial" pixels: absolutely NO change either of av_Z4SM or std_Z4SM for Zland=20m.
GSD=15 m, pan-sharpened. NO smoothing applied. Just the estimated depth is "noisy", as an expression of the reductive assumption on the Soil Line. the pansharpening process : co-registration MULTI/PAN.
PAN solution I suppose that this good result is in part a marked benefit of using the PAN band.
Add heterogeneous atmosphere/water This does not account for the effect of the natural variations of the water/atmosphere optical properties, which can be very nasty in their own right, and increase dramatically deeper than ~half of the shallow depth range.

Profile Land A along the beach Profile Land B through town	Profile Land_C through agriculture
Depth applied to land pixels for three Land profiles ZLand= 1 m ZLand= 2 m ZLand= 5 m ZLand=10 m ZLand=15 m ZLand=20 m ZLand=25 m ZLand=30 m	Average depth retrieved for three Land profiles 0.97+-0.20 m...........N=1124 1.93+-0.23 m 4.83+-0.40 m 9.56+-0.47 m 14.43+-0.47 m 19.34+-0.47 m 24.16+-0.40 m 29.01+-0.27 m.............N=553
Zland=1 m==>0.97+-0.2m	Zland=2 m==>1.93+-0.23m
Zland=5 m==>4.83+-0.4m	Zland=10 m==>9.56+-0.47 m
Zland=15 m==>14.43+-0.47m	Zland=20 m==>19.34+-0.47m
Zland=25m	Zland=30m